Evaluating Multiple Stressor Effects on Benthic–Pelagic Freshwater Communities in Systems of Different Complexities: Challenges in Upscaling

Elisabeth Maria Gross,Franz Hölker,Joey Allen,Herwig Stibor,Mechthild Schmitt-Jansen,Nora Kipferler,Bastian Herbert Polst,Martin Laviale,Gregorio Alejandro López Moreira Mazacotte,Joséphine Leflaive,Vinita Vijayaraj,Sabine Hilt

doi:10.3390/w14040581

Abstract

Upscaling of ecological effects from indoor microcosms to outdoor mesocosms bridging the gap between controlled laboratory conditions and highly complex natural environments poses several challenges: typical standard water types used in laboratory experiments are not feasible in large outdoor experiments. Additionally, moving from the micro- to meso-scale, biodiversity is enhanced. We performed an indoor microcosm experiment to determine the effects of agricultural run-off (ARO) on a defined benthic–pelagic community comprising primary producers and primary consumers, exposed to ambient summer temperature and +3.5 °C. Treatments were replicated in two water types (standard Volvic and Munich well water). We then scaled up to outdoor mesocosms using an ARO concentration gradient and +3 °C warming above ambient temperature, using Munich well water. We included the same benthic macroorganisms but more complex periphyton and plankton communities. All the functional groups were affected by stressors in the microcosms, and a shift from macrophyte to phytoplankton dominance was observed. While effects were present, they were less pronounced in the mesocosms, where a higher biodiversity may have modified the responses of the system to the stressors. The stressor effects observed in controlled experiments may thus be masked in more complex outdoor experiments, but should not be interpreted as “no effects”.

Highlights

Licensee MDPI, Basel, Switzerland.Alternative stable states between phytoplankton and macrophytes commonly occur in shallow lakes, and state shifts are primarily driven by eutrophication [1]
Our results show that micro- and mesocosms can be performed using different water types and still allow comparability among research groups, provided the water type is chemically tested and adjusted for important physicochemical parameters that influence the studied biotic community
These agricultural run-off (ARO) effects were conserved at both scales at ambient environmental temperature, suggesting that ecosystems exposed to a combination of nitrate and pesticides are at risk for shifts to a degraded turbid state, and further action should be taken to reduce the use of these chemicals to protect shallow lakes

Summary

Introduction

Alternative stable states between phytoplankton and macrophytes commonly occur in shallow lakes, and state shifts are primarily driven by eutrophication [1]. The likelihood for such shifts may be enhanced by warming, e.g., through an increased release of nutrients from the sediment [2]. While the focus of many eutrophication studies in shallow lakes has been on phosphorus, recent studies show that high nitrate concentrations may facilitate to regime shifts [3,4,5]. High nutrient input into lakes occurs after heavy rainfall published maps and institutional affil-. A combination of nutrient and pesticide stressors may further facilitate the shift to turbid conditions

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